December 21, 2010

You can fail a radiated susceptibility EMI test if the physical dimensions of the unit under test (UUT) are just right. Such was the case with an aircraft fuel gauge in which the fuel probe's dimensions had been designed to match up with the aircraft's fuel tank.

At almost any irradiation frequency, everything worked okay, but not at 120 MHz.

It turned out that the probe's length was a half-wavelength at 120 MHz and worse still, there was a right-angle joint at the 37% of length point as shown above.

Dimensionally, this fuel probe assembly was a classic Windom antenna at 120 MHz and when it was irradiated at that frequency, the supporting electronics was thrown into all kinds of malfunctions.

None of the probe's dimensions could be changed because of fuel tank restrictions. However, since the fuel tank was metallic and therefore a good RF shield, a waiver was granted because there would never be any 120 MHz irradiation of the fuel probe in actual service.

December 16, 2010

Some troublesome digital chips were not starting up properly if their +5 Vcc was applied with too slow a rise time. (Hey, sometimes things like this can happen, right?) The minimum required rise time for that +5 Vcc rail was determined to be 45 mSec.

This was a problem which later resolved by the chip vendor, but while the problem still existed, a circuit was needed to delay Vcc application to the troublesome chips until the power supply voltage had risen to a particular threshold value, and not before, and to then apply the already risen power supply voltage to the chips with a relatively fast rise-time as sketched here:

December 14, 2010

An equivalent circuit for a quartz crystal has two resonant frequencies, one at a parallel resonance and one at a series resonance, where the parallel resonant frequency is always slightly higher than the series resonant frequency.

Shamelessly setting the component values of this equivalent to yield the proper values of parallel resonance, series resonance and equivalent series resistance of a specific crystal, an impedance calculation versus frequency was performed and an actual crystal was tested with the following results:

December 11, 2010

In the Wednesday, December 8th, B section of the WSJ an article with the title "Electric Vehicles Arrive, Firms See Payback in Trucks" appeared. Parts of the article are repeated in the following.

Staples Inc., Frito-Lay division of PepsiCo, FedEx Corp., AT&T Inc. and a few other companies have begun purchasing electric delivery trucks. Proponents say they make more sense in many ways than electric cars. That's because delivery trucks generally drive short, defined routes each day, which are better suited to the limited range of battery power. Moreover, electric vehicles provide greater savings to companies than to consumers compared with diesel or gasoline models, not only in fuel costs BUT in maintenance.

Staples has ordered 41 trucks from Smith Electric Vehicles of Kansas City, Mo., and will start receiving them in January, There is "a real strong chance we'll make a second order for 40," a Staples spokesman has said.

December 09, 2010

An aluminum electrolytic capacitor that hasn't been in service for a long time may need to be brought back to service by very slowly raising its voltage up to rating. If you try to bring one up too rapidly, the capacitor can be destroyed.

This circuit enabled the resurrection of a very old, spider webbed, dust covered 14000 µF capacitor that had been sitting idle in a storage closet for how long, nobody knew.

The 0.1 µF, the two diodes and the 14000 µF capacitor itself form a half-wave, Cockcroft-Walton voltage doubler circuit. However, because the 0.1 µF is so small compared to the 14000 µF, the voltage rise time across the 14000 µF is something around fifteen minutes for the 60 Hz line frequency.

In use, you apply the input line voltage (120VAC at 60 Hz in my case) and turn the autotransformer up just a little bit. Maybe half an hour later, you raise the autotransformer setting upward a little bit, after another half hour you do it again and so on and so on.

Since the risetime is so slow, you don't need quick reflexes if you happen to raise the autotransformer a bit more than you had intended. You can just lower the setting back down again.

Also, if the 14000 µF were to short circuit itself, the AC line would simply be loaded at most by the 0.1 µF which poses no safety hazard.

Just do be sure to put the capacitor inside of something safe in case the capacitor does fail. An empty metal can might suffice.

December 07, 2010

We may examine the linear properties of an ET1104 opto-isolator as follows.

Note that there was no reason in particular for choosing an ET1104 to examine. There just happened to be one on hand. Also, the 1.33K to the LED was used only because it happened to come to hand at the moment. It could just as well have been a 1K.

December 03, 2010

Sometimes, when we look way back in time, we find the most useful things. I refer to this application note:

I had the pleasure of meeting Jim Hawkins back in 1986. It was both an enjoyable and educational experience. His key point to me that day was from the CDS-2081 document that the volts per mil ratings of silicone rubber or RTV is not a constant as the thickness of the RTV is varied.